Method of making conductive silver images and emulsion therefor

ABSTRACT

A silver bromide emulsion containing cadmium iodide and coated on a substrate is employed to produce a latent silver image thereon. This latent image is developed in a high-resolution pattern developer to produce a silver image which when heated to a temperature of 200* to 450* C. produces an image having an electrical conductivity closely approaching that of bulk silver. The high-resolution pattern developer results in a feltlike structure of silver wires which when heated at higher temperatures decreases in reflectivity.

United States atent Herd et al.

[ Mar. 7, 1972 I72] Inventors: Harold H. Herd, Ossining; Theresa 1. Sun,

Peekskill, both of N.Y.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: Dec. 23, 1968 [21] Appl. No.: 786,366

[52] US. Cl .l ..96/38.4

[51] Int. Cl ..G03c 5/26 [58] Field ofSearch ..117/21l,212,229;96/36.2, 96/384 [56] References Cited UNITED STATES PATENTS 3,109,737 ll/l963 Scott ..96/l08 Takei et a]. ..96/l08 Nawn et al ..96/3 8.4

Primary Examiner-William D. Martin Assistant ExaminerRaymond M. Speer Attorney-Hanifin and Clark and Isidore Match [57] ABSTRACT A silver bromide emulsion containing cadmium iodide and coated on a substrate is employed to produce a latent silver image thereon. This latent image is developed in a high-resolution pattern developer to produce a silver image which when heated to a temperature of 200 to 450 C. produces an image having an electrical conductivity closely approaching that of bulk silver. The high-resolution pattern developer results in a feltlike structure of silver wires which when heated at higher temperatures decreases in reflectivity.

l Claims, N0 Drawings METHOD OF MAKING CONDUCTIVE SILVER IMAGES AND EMULSION THEREFOR BACKGROUND OF THE INVENTION This invention relates to the production of silver images. More particularly it relates to the making of silver images having unique and novel properties.

Normal chemical development of the typical silver halide photographic emulsion produces developed silver images that have little or no electrical conductance and relatively low spatial resolution. A conducting silver image can advantageously be utilized in printed circuit techniques, one of the advantages, for example, flowing from such electrical conductivity being the eliminating of the aluminum layer in photoresists and the establishing of a silver pattern directly upon a wafer. A conducting silver image characterized by high spatial resolution would advantageously lend itself, for example, to the providing of a resistive means for writing nonconducting patterns therein.

Accordingly, it is an important object of this invention to provide a method for making electrically conductive silver images.

It is another object to provide a method for making electrically conductive silver images having high spatial resolution.

It is a further object to provide an emulsion suitable for use in the foregoing methods.

SUMMARY OF THE INVENTION General speaking, and in accordance with the invention, there is provided a novel photographic emulsion comprising silver bromide containing cadmium iodide. Also, in accordance with the invention, there is provided a method for producing electrically conductive silver images comprising using a coating of silver bromide emulsion containing cadmium iodide on a substrate to produce a latent image on the substrate, developing the latent image in a high resolution developer to provide a silver image and heating the silver image at a temperature of 200 to 450 C. to render the silver image electrically conductive.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT An example in carrying out the invention is to form an emulsion made by mixing 12 grams of gelatin, 80 ml. of water, 20 ml. of 50 percent silver nitrate solution, 20 ml. of 37.5 percent potassium bromide solution and 0.366 grams of cadmium iodide. The resulting emulsion is utilized as a photographic coating on a substrate, such as glass, to receive a latent photographic image, the latter image thereafter being developed in a high resolution pattern developer.

In this latter connection, several 2-inch square glass blanks were coated with the above-prepared emulsion and were direct and contact printed with a mask by being exposed for about 50 seconds at 30 centimeters to a concentrated arc lamp. After the exposure, the glass plates were immersed into the developer for about 2 minutes and then fixed by a fixer solution, such as Kodak Rapider Fixer solution, for about 3 minutes to insure that all of the unexposed areas were fixed, i.e., that no silver bromide remained. The developed glasses were then dried and heated at about 450 C. for about a minute. The silver image so developed and heated showed conductivities very close to that of bulk silver.

It was found that rather than heating the developed silver image at 450 C. for about a minute, the images could be heated at lower temperatures for longer periods. Thus, the images could be heated at 200 C. for periods up to 1 hour. The emulsion was also coated on different type substrates, such as silicon dioxide wafers, and the same results were obtained.

It was also found during the heating that if the fixed silver image is heated to about 350 C., a conducting mesh of silver wires is produced with high spatial resolution, i.e., a few hundred cycles per millimeter, such mesh having a feltlike structure. Upon heating the mesh to a higher temperature, the reflectivity of the silver mesh is decreased. It is believed that upon the heating to such higher temperature, the gelatin is evaporated from the surface of the blank and the silver wires are caused to clump together. A typical temperature required to cause such clumping may be one that is slightly higher than that produced by a focused source of temperature, such as a laser beam. Thus, with this fixed silver image, it is possible to ease the temperature locally sufficiently to rearrange the felt structure and hence lower the reflectivity as observed without affecting the surrounding areas.

While the invention has been particularly described with reference to a preferred embodiment thereof, it will be understood by those skilled inthe art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is: 1. A method of making an electrically conducting silver image comprising:

producing a latent photographic image in a coating on a substrate of an unmodified gelatin silver bromide emulsion containing cadmium iodide, there being used as said emulsion, one resulting from the mixing of 12 grams of unmodified gelatin, ml. of water, 20 ml. of 50 percent silver nitrate solution, 20 ml. of 37.5 percent potassium bromide solution and 0.366 gram of cadmium iodide;

developing said latent image in a high resolution pattern developer; and

heating said developed image at a temperature of 200 to 

